Magnetic cores are required for many high-performance inductive components, for example, for transformers, electric motors, electromagnets, and antennas. Magnetic cores made of soft-magnetic materials are used for the purpose of bundling and orienting the magnetic flux and thus effectively guiding it.
A special construction and also the subject matter of the present invention are cores having an air gap in which the magnetic flux thus leaves the magnetic material at least once within the magnetic circuit. In slotted ring band, oval band, and rectangular cores, inter alia, the air gap is small in relation to the so-called iron path length located in the magnetic material, correspondingly, the magnetic path in the magnetic material and thus the core must be curved.
In rod cores, the magnetic core is elongate, the magnetic flux exits at least partially from the rod ends and is returned through the surroundings, the path length in the nonmagnetic material (e.g., air) is longer than in the magnetic material here. In addition, further mixed forms are known (e.g., U-shaped cores). In all of these shapes, the flux not only exits at the core ends (facing toward the air gap), but rather also on the sides. In the implementation of the core made of elongate sheet or fibrous elements observed here, flux thus exits not only from the front sides of the elements, but rather also from the side faces, which results in additional problems in comparison to the known solid, isotropic magnetic materials.
The present invention thus relates to all open magnetic cores (the magnetic flux exits at least once from the magnetic material) made of laminated magnetic elements. For reasons of simple illustration and greatest relevance, rod cores are considered above all in the following. The statements may be transferred to slotted ring band cores, for example, in that the rod core is curved in such a way that the rod ends are opposite one another. Rod cores are used as an antenna core, for example. The magnetic flux is bundled more effectively both in transmission and also reception antennas by them than in air coils.
Such an effective action of a magnetic core in connection with a winding surrounding it is necessary to achieve optimized transmission or reception performance, for example. This may be necessary to transmit information, but also to transmit energy. Specifically, corresponding conductive elements are used both in theft protection, identification, and access systems for information exchange over distances of approximately 5 m, and also for conductive power transmission, for example, battery charging (compare GB 2388715 A) or for power supply of sensors or actuators (compare U.S. Pat. No. 3,938,018).
For functional optimization of an inductive element of this type, in particular optimizing the transmitted power, the design of the antenna and its activation are decisive. The maximum possible flux must be generated in the magnetic core in the area of the antenna, which has at least one magnetic core and one winding. Hysteresis losses in the core or losses in the antenna coils due to resistive power, proximity effect, etc., are to be minimized to optimize the efficiency as a result. Especially hysteresis losses would also result in intrinsic heating of the magnetic core in addition to reduction of the transmission power, which may result in damage to the winding or other components in its surroundings.
A magnetic core of this type, as is required for an antenna, is typically constructed as a rod core in the form of a cuboid, which is enclosed by one or more coils. The flux exits from the front faces in the direction of the longitudinal axis of the cuboid in greatest part, but partially also from the side faces and especially at the edges of the core ends. There is typically a flux concentration and thus overload by saturation in these areas of the corners and edges at the core ends. For this purpose, beveling the edges is known as a solution (EP 762535 B1).
For example, ferrites or soft-magnetic metals are known as a material for the magnetic core from the prior art. Materials of this type are typically homogeneous and isotropic, so that the permeability is a scalar and not a second rank tensor. This means that the flux propagates linearly in the magnetic core and corresponding to a field course expected in air. Magnetic cores made of thin layers of soft-magnetic strips, as are often used for cores having an air gap and more recently also for cores of antennas, in contrast, have properties deviating therefrom because of their anisotropy. In particular in regard to the losses due to eddy currents, in the event of a flux in the longitudinal direction of a strip of this type, a reduction of the eddy current strength results in that only a very restricted space is available perpendicular to the flux direction due to the low thickness of the strip. Eddy currents may only propagate very weakly because of this. Only the magnetic flux entering perpendicular to the flat side of a magnetic strip of this type may generate eddy currents in the plane of the strip to a significant extent.
Typical hysteresis losses may be described in metallic material in the frequency range to be observed here, which is between 15 and 150 kHz, for example, by the formula P˜B2 f2 d2. (B=induction amplitude, f=frequency, d=spatial dimension, i.e., the smallest diameter of the eddy current path). The analysis of this formula results in the insight that on one hand the induction amplitude must be distributed as uniformly as possible over the core cross-section and on the other hand the dimension of individual magnetic elements perpendicular to the flux course must be as small as possible.
Therefore, to optimize the magnetic core exploitation, it is to be ensured that the largest possible part of the magnetic flux actually propagates within the magnetic core in the longitudinal direction of the strips or also corresponding rod-shaped magnetic elements. This is counteracted by the effect that in the above-mentioned typical cuboid core shape, the magnetic flux does not run parallel to the antenna axis everywhere. For example, in a typical dipole field, flux lines also enter in the lateral faces of a magnetic core, which correspondingly have components perpendicular to a magnetic element in the form of a magnetic strip or a magnetic rod and/or its longitudinal axis and accordingly generate higher losses.
It initially has a positive effect that the magnetic flux predominantly stays inside the core in a single magnetic element due to the implementation of air gaps or insulation layers between the individual magnetic elements, which is fundamentally desired. However, due to this effect, because of the flux lines entering in the lateral faces of the magnetic core, an increased flux accumulates in the outermost layers of magnetic elements, which may result in overload due to saturation there. Because the hysteresis losses are a function of the square of the induction amplitude and are thus not linear, an avoidable increase of the loss rate results through such an uneven flux distribution over the cross-section of the magnetic core. A further object thus results of distributing the magnetic flux as uniformly as possible over the cross-section of the magnetic core.
This is desirable in particular if an asymmetrical form of the magnetic flux arises due to the presence of magnetically active parts in the surroundings of the magnetic core, such as metal sheets, which may displace the magnetic flux, or soft-magnetic materials, which attract the magnetic flux. An asymmetrical form of this type inside and outside the core is also to be taken into consideration by a corresponding design of the magnetic core.
In ferrites, on one hand such anisotropy effects do not result because of the isotropy of the material, on the other hand hysteresis losses due to eddy currents are not especially pronounced at all because of the high specific ohmic resistance.